The western Sea of Japan is a region in which mesoscale cyclones are frequently observed as cold polar air breaks out over the sea. In this region, mesoscale cyclones occasionally develop along a convergence zone that forms on the lee side of the mountains north of the Korean Peninsula. A meso-α-scale cyclone (MαC), in which two meso-β-scale cyclones (MβCs) were embedded, was observed in the western Sea of Japan on 23 January 1990. This structure is referred to as a “multi-scale structure” of a mesoscale cyclone. A simulation experiment and a number of sensitivity experiments with respect to several forcing factors were performed to elucidate the development mechanism of mesoscale cyclones. Stably stratified air flows around the mountains north of the Korean Peninsula and the convergence zone forms on the lee side of the mountains. Large amounts of sensible heat and latent heat are supplied from the sea to the atmosphere. The diabatic heating due to condensation and vertical diffusion of the sensible heat and horizontal advection of the potential temperature θ are almost in balance with the negative vertical advection of θ. This results in an intense upward motion in the convergence zone. Since baroclinicity is intense in this region, an upward transfer of the horizontal momentum around the convergence zone intensifies the upper-level (800∼600 hPa) divergence. Consequently, the surface pressure decreases, and the MαC develops. The upward motion and vorticity are concentrated mostly in the MβCs, which are considered to be the cores of the MαC. On the other hand, the MαC provides an environment for the formation and development of the MβCs and affects their movements.
We have developed a disposable type of radiometer-sonde for independently measuring the downward and upward broadband fluxes of solar and terrestrial radiation with a pair of up-looking and downlooking shortwave (SW) and longwave (LW) radiometers. The radiometer-sonde contains a built-in rawinsonde for simultaneously measuring air-temperature, humidity and wind profiles. The paper outlines the state-of-the-art of the radiometer-sonde, and discusses its performance. We have investigated such characteristics as temperature dependence, linearity, and transitional thermal effects of the SW radiometers, as well as transitional thermal effects of the LW radiometers through laboratory experiments and sonde-launching tests. Further, we have tested the practical performance of the radiometer-sonde for several cases of clear skies. The measured flux profiles were compared with those computed for modeled atmospheres. The developed radiometer-sondes have proved their adequate performance to simultaneously measure the radiative flux profiles and the atmospheric profiles with enough accuracy to detect radiative effects of aerosols (SW radiometers) and water vapor (LW radiometers) in the lower troposphere. It is also suggested that the SL radiometer-sondes will be also effective for measuring the radiative flux profiles in high-level ice-clouds with stronger radiative effects.
In the Japanese Cloud and Climate Study (JACCS) cirrus experiment, simultaneous measurements of cloud radiative and microphysical properties were conducted by using the combined-sonde (radiometersonde + hydrometeor-video-sonde (HYVIS)) observation system at the Meteorological Research Institute (MRI), located at (36.05°N, 140.13°E) in Tsukuba, Japan, during early summer seasons from 1995 to 1999. We have analyzed the radiative properties of frontal ice-clouds observed by the shortwave and longwave radiometer-sondes (Asano et al. 2004). To interpret the observed radiative flux profiles, we have also performed radiative transfer calculations for horizontally homogeneous atmospheric models, where the single-scattering properties of ice-clouds were computed by anomalous diffraction theory for ice-crystals observed by HYVIS. On an average of the observed frontal ice-clouds, the shortwave re- flectance, transmittance and absorptance were estimated to be 0.41±0.03, 0.51±0.06, and 0.08±0.09, respectively, for the averaged ice-cloud layer with a mean visible optical thickness of 4.6 and a mean geometrical thickness of 5.4 km (mean volume extinction coefficient of 0.85 km-1). The ice-clouds were significantly heated by absorption of solar radiation in daytime. On the other hand, the mean effective emittance was estimated to be about 0.86±0.37, showing that the frontal ice-clouds never acted as blackbody for longwave radiation. The lower parts of ice-cloud layers were heated by absorption of longwave radiation from the surface and the atmosphere below the ice-clouds, while the upper parts were cooled by emission of longwave radiation to space. The shortwave and longwave heating profiles could make the daytime ice-cloud layers thermodynamically unstable.
Using the twice-daily data for the 10-year period from 1989 to 1998, from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, the authors analyze the climatological Rossby wave sources at 200 hPa in summer, focusing on the uppertropospheric circulation related to the subtropical convergence zones (STCZs) in the Northern Hemisphere, namely, the North Pacific convergence zone (NPCZ, including the Meiyu/Baiu front) and the North Atlantic convergence zone. In the June-July-August (JJA) period, a Rossby wave sink clearly appears along the uppertropospheric westerly jet streams over East Asia and the extratropical North Pacific, but is very weak in the extratropical North Atlantic. For the JJA mean, or in early summer (June), the Rossby wave sink over East Asia, and the extratropical North Pacific, is much stronger than that in the tropical western North Pacific. The latter sink, however, becomes stronger in late summer (August), corresponding to the enhancement and poleward shift of the atmospheric convection, and becomes comparable with the former sink. The components of the Rossby wave source, namely, vortex stretching and advection of absolute vorticity by divergent flow, are also examined and used to judge the roles of tropical and extratropical heatings. Despite the integrated appearance of the sink along the NPCZ, this sink results dominantly from the advection of absolute vorticity by divergent flow in East Asia, and the western Pacific, and from vortex stretching over the mid-Pacific. These results reveal that the tropical heating is responsible for the maintenance of upper-tropospheric circulation in East Asia, and the extratropical western North Pacific, through the advection of absolute vorticity by divergent flow, while the diabatic heating in the NPCZ plays a dominant role in maintaining the upper-tropospheric circulation over the mid-Pacific.
Improvement in forecast accuracy is expected to result from the incorporation of rain type and precipitation retrieved from the TRMM Microwave Imager (TMI) into numerical weather prediction (NWP) models, especially in the tropics, where latent heating is the main energy source of atmospheric motion. A One-dimensional Variational Method (1DVAR) was developed in the present study for the assimilation of convective and stratiform rain flags, and precipitation retrieved from TMI into a Japan Meteorological Agency (JMA) global NWP system. In order to simplify the observational operators of the 1DVAR, it was assumed that the rain flags (precipitation) were functions of total water content (divergence) alone. Using these operators, the 1DVAR sought the optimal total water content (divergence), by minimizing a cost function that consists of a background total water content (divergence) term, and observation terms for the retrieved rain flags (precipitation). The impact of the TMI rain flags and precipitation assimilation on the NWP forecasts was examined in a case study of meso-scale precipitation systems over the Indian Ocean around 06 UTC 16 July 1998. The 1DVAR made deep (mid-tropospheric) moist layers with strong (moderate) updrafts in the TMIretrieved heavy rain areas (stratiform rain areas). This assimilation enabled the NWP to spin-up heavy precipitation in correspondence with the TMI retrievals, and to maintain the heavy rain areas for 24 hours. The creation of deep moist layers was essential to maintain these heavy rain areas. The assimilation also induced model stratiform precipitation around the TMI-retrieved stratiform rain areas, although the model rain areas lasted for the first several hours only. The impacts of the assimilation were also examined by including the 1DVAR into the forecastanalysis cycle of the global NWP system for July 1998. The results show that the assimilation improved large-scale patterns of precipitation, convective and stratiform rain areas, and precipitable water content over tropical oceans. It was also found that the assimilation changed double-peak profiles of diabatic heating around inter-tropical convergence zones, into more realistic mid-tropospheric peak profiles. This change was mainly ascribable to the enhancement of tall convection and suppression of low-level convection by the 1DVAR. The Hadley circulation was strengthened (weakened) in the upper to mid- (lower-) troposphere in accordance with the changes in diabatic heating.
Hailstorm is a rather unusual weather phenomenon in Taiwan particularly in the summer season. The hailstorm event occurring over northern Taiwan on 2 July 1998 was investigated using conventional data, Doppler radar observations, and satellite cloud winds. Results showed that an upper-level cold vortex provided favorable conditions for the development and evolution of the storm. The storm appeared to be triggered by the low-level convergence associated with local circulations coupled with the upper-level divergence forced by a jet streak of cold vortex. Backing of winds and vertical shears with height provided by cold vortex appears to be instrumental for the westward propagation and intensi- fication of the convective system. A couplet of mesoscale vortices was observed in the low- and midtroposphere and was found to be generated by the tilting process on the vertical wind shear through a strong updraft in the convection.
Summer weather in northern Japan since 1982 appears to exhibit a distinct five-year cycle, with the pressure difference between Wakkanai and Sendai (PDWS) as an index of this climatic variability. The temporal variation in the June—August mean PDWS in the period 1982 to 2001 progresses through four cycles, each with conspicuous similarities, such as a peak PDWS in the second year of each cycle. The temperature also progresses in similar five-year cycles, varying inversely to the trend in the PDWS. The pattern of mean surface-pressure anomalies in the second year of each cycle is typical of the Yamase, a cold northeasterly wind, and is consistent with the cool summer in northern Japan. The following year of each cycle is characterized by a prevailing subtropical high-pressure cell, consistent with the hot summer. The correlations between the seasonal variations in the PDWS, and sea-surface temperature indicate that in the summer following an El Niño event, the PDWS tends to become positive and a cool summer ensues. An area of strong positive correlation between the PDWS and the 500 hPa geopotential height, can be identified extending from Southeast Asia to east of the Philippines, appearing to be closely related to the convective activity around the western tropical Pacific. The progression of the difference in sea-surface temperature between the South China Sea, and east of the Philippines, exhibits similar temporal variations to the PDWS, and may play an important role in the sudden change from cool summer to hot summer in northern Japan.
A line-shaped convective system (LCS) was observed on 19th June 2001 in southern Kyushu, Japan in a field experiment called X-BAIU-01. Operational meteorological radar observed a mesoscale line of precipitation in this experiment. A detailed analysis of the radar observation indicated that this line of precipitation had a structure of the LCS, which is a type of mesoscale convective system (MCS). The convective cells in the LCS propagated east-northeastward at the speed of 25 m s-1. The LCS formed between two wind profilers. “Dual wind profiler” analysis illustrates mesoscale-flow patterns upstream of the LCS, such as horizontal convergence with upward airflow above 500 m in altitude, and divergence with downward airflow underneath. Moreover, an integrated analysis of the dual wind profiler, operational weather radar and radiosonde measurements revealed the formation of the LCS as follows. The surface atmosphere from the ocean climbs a mountain slope in southern Kyushu up to the lifting condensation level (LCL) by environmental wind, and forms a stratus cloud. The atmosphere continues climbing the mountain slope up to the height of a convergence layer aloft. The strong updraft in the convergence layer changes the type of cloud from stratus to cumulus at this height, and increases precipitation intensity. The atmosphere continues ascending up to the level of free convection (LFC) in the updraft region. After reaching the LFC, the atmosphere increases the ascending speed by buoyancy inside, and develops convective cells. A number of cells are advected by the environmental wind and form the LCS. This process indicates that this type of the LCS does not have a back-building structure, which is one type of the LCS, but has a terrain-induced structure. A refined conceptual model of this type of the LCS is presented based on previous studies and observations reported in this paper. The analysis also indicates that the isolated mountains or islands upstream are not essential for the formation of this type of the LCS, because the profile of the atmosphere does not satisfy the conditions necessary to trigger convections or to excite lee waves by the mountains or islands.
We investigated the effects of large-scale orography on the tropical coupled atmosphere-ocean system over the Indian and Pacific Oceans in northern summer, using the Meteorological Research Institute coupled atmosphere-ocean General Circulation Model (GCM). Six different experiments were conducted with mountain heights of 100%, 80%, 60%, 40%, 20%, and 0% of the standard mountain height. The results show that a pool of warm sea surface temperatures (SSTs) appears in the western Pacific as orography increases, although SST in the tropical Pacific decreases as a whole. In addition, easterly winds at low levels over the equatorial Pacific strengthen as mountains rise. The enhanced easterlies alter surface heat flux and ocean dynamics, changing the water temperature field in the upper Pacific Ocean. Water temperatures between the surface and 300 m in the western Pacific increase as upwelling is suppressed and the thermocline deepens. Water temperatures in the eastern Pacific decrease and the thermocline rises. Therefore, the east-west gradient of water temperature in the Pacific is enhanced for cases with mountain heights of 80% and 100% of the standard mountain height. In the equatorial Indian Ocean, the east-west gradient of ocean heat content weakens as mountain heights increase, in connection with the evolution of the Asian summer monsoon. An increase in diabatic heating over South Asia as mountain heights increase causes sea level pressure (SLP) to decline over the Indian Ocean, and enhances upper atmospheric divergence over the eastern hemisphere. Consequently, the east-west circulation over the Indian and Pacific Oceans strengthens as mountains become taller. The east-west circulation may also be enhanced by changes in convective activity associated with SST changes. The coupled general circu- lation model (GCM) results show that uplift of large-scale orography, particularly the Tibetan Plateau, significantly affects the tropical atmospheric and oceanic climate, by changing the east-west circulation and altering the evolution of the Asian summer monsoon.
Based on the International Satellite Cloud Climatology Project (ISCCP) data, and the World Meteorological Organization (WMO) surface synoptic observations, spatial distributions and seasonal variations of total cloudiness and fractional cloud amount of high, middle and low clouds over China are examined. It is found that low clouds mainly appear along the southeast coast of China, middle clouds dominate southern China and high clouds mainly occur over northern China. Seasonal variations of convective clouds over northern China, southern China and the Tibetan Plateau are similar, reaching a maximum in summer and a minimum in winter; whereas the seasonal variation of stratiform clouds displays large spatial variation, with opposite phase in northern and southern China.
This is the first part of a two-part study that seeks links between summer rainfall variability in China/Japan and the large-scale circulation over the East Asia/western Pacific region, in both space and time, for the period of 1951-2000. Part I focuses on the spatial patterns, while Part II on the dominant timescales with which some extremely wet and dry summers in these two countries may have occurred. In this part, we use the singular value decomposition (SVD) method to find the dominant covariance patterns of summer rainfall anomalies over 160 stations in China and 72 stations in Japan, and the regional 500 hPa geopotential height anomaly over 60°E-160°W, 20°S-70°N. The associated 850 hPa horizontal wind patterns are studied by linear regression against the temporal coefficients of these modes. For a positive temporal coefficient, SVD1 represents a north-south wave pattern: a blocking high over eastern Siberia, a southwestward advanced and intensified subtropical high, and an elongated mid-latitude low in between. This mode mainly represents wet/dry trends over the 50-year period in the following regions: wetter summers in the Yangtze River valley and southwestern Japan, and drier summers in northern China and the Kinki area of Japan after the climate regime shift in the late 1970s. SVD2 and SVD3 are a pair of north-south wave patterns over the East Asia/western Pacific region with their phases in quadrature. The dominance of SVD2 or SVD3 implies a shift of the main rain band in China across the Yangtze River in the north-south direction, in association with very different rainfall patterns in Japan—either most of Japan experiences above- or below-normal rainfall in the same summer due to SVD2, or rainfall anomalies are opposite in sign between the Pacific side and the Sea of Japan side due to SVD3. These three dominant SVD spatial patterns, with their positive and negative phases, represent six dominant patterns of the western Pacific subtropical high and the mid-latitude wave systems in the regional large-scale circulation, in association with six main rain patterns in the two countries. The temporal behaviors of these modes will be further studied with wavelet and composite analyses in Part II.
Based on the results from the SVD analysis in Part I, here we use wavelet and composite analyses to identify the dominant timescales on which the extremely wet and dry summers in China/Japan, influenced by the large-scale regional atmospheric circulation, may have occurred in 1951-2000. An apparent trend is found in the temporal coefficient of SVD1, with a less significant trend in SVD2. Both trends change sign in the late 1970s. Associated with these trends, the 500 hPa continental high over Eurasia is greatly intensified, with increased 850 hPa northerly wind trends between 100°E-120°E. This circulation change is associated with wet trends in the Yangtze River valley, northern Kyushu and the Pacific side of Tohoku, and dry trends in northern and southern China, Kinki and the Nansei Islands of Japan. All three modes display interdecadal variability, but their cycle lengths and intensities change considerably before and after the late 1970s. The quasi-biennial (QB) signal dominates over the ENSO signal on interannual timescales in all three modes, except for SVD3 after the late 1970s. In SVD2, the QB signal is predominant and modulated on bidecadal and longer timescales. The QB's amplitude peaks in the 1990s. During this decade, the extremely wet and dry summers occurred alternately in the two countries are mainly on the QB timescale of SVD2, but also modified by other timescales and other modes. Our results suggest that the timing and the location for an extremely wet/dry summer to occur be largely determined by the interaction and reinforcement among the dominant timescales of the dominant SVD modes. Since the regional large-scale circulation is better predicted at one season lead than summer rainfall, the spatial and temporal relationships found in this work may help explore possible deterministic causes of spatial/temporal covariability in circulation and rainfall, leading to improved regional climate forecast.
An analysis of the diurnal variation of precipitable water and convective activity was conducted for sunny summer days around Mt. Tanigawa. Precipitable water and convective activity exhibited a pronounced diurnal variation, with dual maxima in the daytime (10-14 JST), and in the evening (18-22 JST), and these maxima of convective activity almost coincided with maxima of precipitable water. The mechanism responsible for the diurnal variation with dual maxima of precipitable water and convective activity, was discussed using GPS network data, and the findings of Iwasaki and Miki (2001 and 2002). The process of diurnal variation with dual maxima of precipitable water, around Mt. Tanigawa can be divided into 4 periods. 1) Late night to early morning: Precipitable water decreases due to large scale subsidence associated with a Pacific High, and the large scale subsidence persists throughout analysis period even in the daytime; 2) Early morning to noon: As a valley-wind circulation develops, moisture transport from the semi-basin (valley) to the mountain also becomes active. Because the effect of the moisture transport exceeds the effect of the large scale subsidence, precipitabe water around Mt. Tanigawa begins to increase gradually; 3) Noon to around 15 JST: Because the atmosphere in the valley becomes dry due to the compensating downdraft of the valley-wind circulation, the moisture transport from the semi-basin to the mountain decreased. The effect of large scale subsidence exceeds the effect of the moisture transport, so that precipitable water decreases in the daytime; and, 4) Around 15 JST to late night: Precipitable water is recovered due to moisture convergence from the socalled “extended sea breeze,” and moisture advection from the mountains, located on the windward side of Mt. Tanigawa. An increase of precipitable water in the daytime, and the evening, must work to unstabilize atmosphere, and surface heating, due to solar radiation that enhances the instability in the daytime. These effects contribute to formation, and/or development, of Cb groups in the daytime and the evening around Mt. Tanigawa.
A numerical simulation of a sea-breeze front (SBF) is carried out to investigate the nocturnal frontogenesis (FG) phenomenon that occurs as the SBF propagates inland during the nighttime. At first, we reproduce the FG event of the nighttime SBF based on high-resolution numerical modeling. Then, to reveal its formation mechanism, the FG equation is used, and contributions of each term in the FG equation are estimated for explaining this nocturnal FG event. The results show that the nocturnal FG event of the nighttime SBF: (1) is mostly attributed to the confluence effect due to the horizontal velocity convergence and intensification of the horizontal potential temperature gradient at the front; (2) is partly attributed to the no significant tendency of the turbulence effect associated with the surface cooling.